Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Some vehicles employ electric motors used in combination with the internal combustion engine to decrease fuel consumption. For example, an electric motor may be used to recapture energy that would otherwise be converted to heat during braking and then use that energy to reduce the power demand on the internal combustion engine. Also, the motor may supplement the power of the internal combustion engine during acceleration events such that a smaller, more efficient engine may be installed without sacrificing performance.